How to store the state of a panel before the form is minimized and restore on maximize

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I have a drawing application. The application is based on the .NET core framework and I happen to have a panel on my form where I draw several shapes such as line segments, circles, rectangles and polygons. I have noticed quite a bizarre behavior when I minimize the form, all my drawings disappear on maximizing again. How can I store the state of the panel in some variable and restore the state of the panel when the windows is maximized again?

What I have tried:

I have the code snippet below that listens when the form size is changed and should store the state of the panel and restore on maximize, how to do this is the problem

Posted 31-Mar-23 5:32am

Tim the Gamer

Updated 31-Mar-23 6:14am

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0x01AA 31-Mar-23 11:46am

My guess is: If you draw your shapes etc. in Panel.Paint Event you should be fine.

Tim the Gamer 31-Mar-23 11:49am

My code does not use the paint event handler method of the panel to draw components on it. I listen on the click events the user performs on the panel such as holding down mouse and moving it around or clicking two points and then checking the line button. Will the paint event let me choose a specific shape then call it to draw that shape?

pdoxtader 31-Mar-23 12:12pm

The paint event is the standard place to do your graphics / drawing in winforms. When I do that kind of thing, it goes in the paint event. You'll trigger updates to your panel using .Refresh() or Invalidate(), depending. You may need to create some drawing operation objects and add them to a list that gets enumerated and executed during paint.

[no name] 31-Mar-23 12:04pm

WPF, UWP, and Windows Forms can maintain visual "state" (using "their" Windows and Pages). If you don't use their facilities, you have to "repaint" yourself. Or, you're accidently clearing your own work through some event.

Tim the Gamer 31-Mar-23 12:15pm

@Gerry Schimtz, no I have minimized the form several times and the panel comes back with all the drawings gone. That is why I needed to learn if there is a way to store the panel state on minimize and restore on maximize unless we can invoke the Panel Paint event handler from the other mouse click event handler where I am listening for clicks and then check the tag and draw the appropriate shape, I also get the compile time error that cannot convert EventArgs to MouseEventArgs

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OriginalGriff · Accepted Answer · 2023-03-31T06:14:00

Solution 1

Unless you draw on a persistent surface (a bitmap for example) or store the actions that you do draw, all such drawings on your panel are transitory - they will be lost the next time a Paint event is triggered for all of part of your panel.
And that includes minimize, maximize, resize, or the panel being partially or totally covered by a different control or other window.

For editable persistent objects, you need to store the "draw line from (x1, Y1) to (X2, Y2)" and suchlike in a collection, and handle the Paint event to iterate that collection and draw from there. When the user adds a new instruction, you add it to the collection and call Invalidate on the Panel which causes a Paint event to be triggered.

Posted 31-Mar-23 6:14am

OriginalGriff

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Tim the Gamer 31-Mar-23 12:25pm

Drawing the actions is the next part of the project and I can use a Queue to store the actions so that the user can undo and redo painting steps. Is there a way I can call the Paint Event handler of the panel with a tag specifying the kind of shape to be drawn

OriginalGriff 31-Mar-23 12:31pm

The kind of shape is what you store in the Queue ... create an abstract DrawItem class, and derive your various items from that, implementing an abstract Draw method that takes a Graphics context.
That way, the Paint handler just iterates the collection and the items draw themselves.

Tim the Gamer 31-Mar-23 12:34pm

Okay thanks I will write the class and post a new comment in case I need any issue clarified. You also mentioned using a bitmap, I can actually save the drawings on the panel as a bitmap and redraw them on the panel when the form is maximized again.

OriginalGriff 31-Mar-23 12:51pm

You could, but if you are planning on allowing edits later, remember that you can't "undo" drawings on a bitmap - just clear and restart.

Tim the Gamer 31-Mar-23 12:59pm

That's the problem with using a bitmap, Undo won't work which can be super annoying. Let me implement the abstract classes and methods which would need inheritance for the various shapes, should I require the graphics context from the constructor or the method?

[no name] 31-Mar-23 13:22pm

You use a "writeable" bitmap and shadow the pixel buffer with you own arrays / objects. (I use one for a "mini-map" of my main zoomed "map"). No lag. (async writes)

OriginalGriff 31-Mar-23 13:57pm

Get the Graphics context from the PaintEventArgs in the Paint handler - that way you aren't responsible for destroying it when you are done - and it makes your code more "portable" as the same code will work unchanged for a printer.

Tim the Gamer 31-Mar-23 13:27pm

I already used the function graphics. CopyFromScreen to grab the drawings from the panel and save to a bitmap. The drawings don't dissappear but I will need to implement the solution suggested here for the undo and redo functionalities.

Tim the Gamer 31-Mar-23 14:06pm

Okay I will use Graphics property of the paint event object of the panel. Should I initialize it at the top of the class then assign it inside the event handler and then use it to draw shapes on the panel?

OriginalGriff 31-Mar-23 14:27pm

No! It is created for you when your Paint even handler is called, and is ready to rock'n'roll - that's the whole idea: the system knows what you need to draw onto, and you don't need to!

This way, you can change the target control from a specific panel to any control - so you can even have two on the same form if you need it (maybe a "Current" and a "Preview" display for example). Without changing your code!

Tim the Gamer 31-Mar-23 14:30pm

Okay thanks, let me get to it then.

Tim the Gamer 3-Apr-23 3:13am

Hey OriginalGriff, this type of object works for both the undo and redo events Queue<bitmap> to undo call. Dequeue which removes the bitmap at the top of the queue and returns the nsw bitmap at the top.